Scalar Scatter Plot (#4616)

* add failing test

* pass test

* initial review actions

* test 2nd arg scalar

* whatsnew

docs/src/whatsnew/dev.rst

@@ -41,6 +41,9 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ reverted part of the change from :pull:`3906` so that
    :func:`iris.plot.plot` no longer defaults to placing a "Y" coordinate (e.g.
    latitude) on the y-axis of the plot. (:issue:`4493`, :pull:`4601`)
+
+#. `@rcomer`_ enabled passing of scalar objects to :func:`~iris.plot.plot` and 
+   :func:`~iris.plot.scatter`. (:pull:`4616`)
 
 
 💣 Incompatible Changes

lib/iris/plot.py

@@ -652,13 +652,13 @@ def _get_plot_objects(args):
         u_object, v_object = args[:2]
         u, v = _uv_from_u_object_v_object(u_object, v_object)
         args = args[2:]
-        if len(u) != len(v):
+        if u.size != v.size:
             msg = (
                 "The x and y-axis objects are not compatible. They should "
                 "have equal sizes but got ({}: {}) and ({}: {})."
             )
             raise ValueError(
-                msg.format(u_object.name(), len(u), v_object.name(), len(v))
+                msg.format(u_object.name(), u.size, v_object.name(), v.size)
             )
     else:
         # single argument

lib/iris/tests/unit/plot/test__get_plot_objects.py

@@ -0,0 +1,45 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+"""Unit tests for the `iris.plot._get_plot_objects` function."""
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests  # isort:skip
+
+import iris.cube
+
+if tests.MPL_AVAILABLE:
+    from iris.plot import _get_plot_objects
+
+
+@tests.skip_plot
+class Test__get_plot_objects(tests.IrisTest):
+    def test_scalar(self):
+        cube1 = iris.cube.Cube(1)
+        cube2 = iris.cube.Cube(1)
+        expected = (cube1, cube2, 1, 1, ())
+        result = _get_plot_objects((cube1, cube2))
+        self.assertTupleEqual(expected, result)
+
+    def test_mismatched_size_first_scalar(self):
+        cube1 = iris.cube.Cube(1)
+        cube2 = iris.cube.Cube([1, 42])
+        with self.assertRaisesRegex(
+            ValueError, "x and y-axis objects are not compatible"
+        ):
+            _get_plot_objects((cube1, cube2))
+
+    def test_mismatched_size_second_scalar(self):
+        cube1 = iris.cube.Cube(1)
+        cube2 = iris.cube.Cube([1, 42])
+        with self.assertRaisesRegex(
+            ValueError, "x and y-axis objects are not compatible"
+        ):
+            _get_plot_objects((cube2, cube1))
+
+
+if __name__ == "__main__":
+    tests.main()